Finally had a chance to check this out - super cool! I'm not well-versed in Live2D yet, so this is great to see. One suggestion I have from playing around a bit - you can get a nice open-close loop with the formula math.sin(st * LIP_FLAP_SPEED_FACTOR)*0.5+0.5 in the get_mouth_open_value function (assuming you import math first). Then it's a smooth transition back to closed after the mouth is open. Will be keeping an eye on this method for future Live2D projects!